1. Field of the Invention
The invention relates to apparatus for measuring the diameter of the electron beam of a scanning electron microscope (SEM). More particularly, the invention relates to the use of a heated wire acting as a knife edge for measuring the electron beam diameter and a transducer assembly for providing a known reference distance against which the measured electron beam diameter is compared.
2. Description of the Prior Art
The method currently used to measure the resolution of an SEM is to make a micrograph at high magnification of an object which contains a wealth of high contrast and fine detail. A measurement is then made of the shortest distance which is resolved on the micrograph and converted via the known magnification into a distance resolved on the specimen. Since the smallest resolvable distance is approximately equal to the beam diameter, measurements of the resolution have been used to determine beam diameters. The difficulty with this approach comes from the fact that the result depends upon the interaction between the specimen and the electron beam. The technique is also very slow. There is no way it could be used to determine the optimum settings for a combination of the variables that determine the beam diameter.
Another method has been proposed in an article "SEM Parameters and Their Measurement," D. C. Joy, SCANNING ELECTRON MICROSCOPE, 1974 (Part I) Proceedings of the Seventh Annual Scanning Electron Microscope Symposium, ITT Research Institute. The method uses a thick, rectangularly-shaped knife edge positioned at the narrowest part (i.e., focal point) of the electron beam. An aperture and electron detector below the knife edge are used to collect the current from the electron beam. The electron beam is scanned across the knife edge and the current density received by the electron detector is plotted on an x-y graph to give an indication of electron-beam diameter.